The Sense Organs of Snails

Sense organs are special organs, specifically acquiring information from the
environment and sending them on to the nervous centre in the form of electrical
impulses. Snails do not have a brain in the strict sense of the word, but in
most groups the neural knots (ganglia) are quite concentrated in a ring around
the oesophagus near the head, also referred to as the buccal ring.

Acquiring and processing of sensual stimuli in snails only partially takes
place using specialised sense organs. Especially smell, taste, humidity,
temperature and touch are sensual information that is acquired by sense cells
dispersed over the snail's outer skin. Those sense cells are especially
concentrated on the head, the tentacles and the
lips. Apart from their original task as tactile organs, a snail's tentacles and
lips also carry sense cells of smell and taste.

Sense organs according to the definition given in the beginning, in snails
only are present in three different types: The eyes, the equilibrium organ and
the so-called osphradium in gill-breathing snails.

Eyes

In the class of snails (Gastropoda) almost all thinkable stages of eye
evolution are present. While ground-living limpets (Patellidae)
are equipped sufficiently with pigment cup eyes, in herbivorous
ormers (Haliotidae) and
top shells (Trochidae)
have developed into pinhole camera eyes. Closed vesicular eyes with a vesicle (bubble)
of a refractive gelatinous secretion have developed in predatory marine
gastropods, but also in freshwater gastropods. This gelatinous bubble has
solidified to form a lens especially during the evolution of terrestrial snails,
especially in the terrestrial pulmonate snails (Stylommatophora).

Apart from their eyes as specialised sense organs, snails also have light
sense cells in other parts of the body. Dispersed over the outer skin, those
sense cells enable the snail to display a shadow reflex - a protective reaction
to a shadow falling suddenly over the snail, which in nature usually means a
predator.

Statocysts

The equilibrium organs (statocysts) of snails are capsules or vesicles filled
with liquid, calcareous grains (statoliths) floating within. As soon as the
snail moves or changes its position, inertia has the statoliths float against
the statocyst's wall. There, sense hair or cilia detect the contact which is
then processed to the nervous system and the snail receives information about
its position in space.

Snails' statocysts sit upon the pedal ganglia or the pleuro-pedal connective
(the nerve between pleural and pedal ganglion), but they are innerved by the
cerebral ganglion.

Osphradia

The osphradium of gastropods is a specialised chemical sense organ, which
originally is present twofold in the pallial cavity. So, while
limpets (Patellidae) still possess two
osphradia, the left osphradium is reduced in more advanced gastropod groups as a
consequence of torsion.

One of the osphradium's tasks is to examine water quality, so that the snail
occasionally is able to close the mantle cavity at the right time.
Mud snails (Viviparus) also
use their osphradium to find partners.

Carnivorous snails, such as the common
whelk
(Buccinum undatum), use their osphradium to find prey. To do so, they
perform searching movements with their sipho to find its scent. On the other
hand, even the usually quite stationary limpets flee, as soon as they detect a
predator's (such as the netted dog
whelk,
Hinia reticulata) scent in their vicinity.

Netted dog snails again only flee from sea stars they recognize as dangerous
(not all sea stars are) by their scent.

The osphradium, probably together with the gill, has been reduced during the
evolution of terrestrial life in snails, apparently because the main part of the
osphradium's task is achieved by examining the water flowing through the pallial
cavity. Obviously, the air, pulmonate snails breathe cannot be examined that
way. For that reason, terrestrial snails' tentacles,
especially the small ones, have developed a special proficiency in detecting
scents.